[time-nuts] Oscilloscope-based measurements of frequency stability

Chris Burford cburford1 at austin.rr.com
Mon Oct 1 09:20:30 EDT 2018

This sounds interesting enough and I would appreciate any notes or insight on doing this. I have a PRS10 and several GPSDOs that I would like to evaluate for performance on my scope.

Many thanks.

---- Dana Whitlow <k8yumdoober at gmail.com> wrote: 
> I cheered when I saw Dave B's "silly question", for
> then I realized that I'm not the only one who likes
> to measure things with an o'scope.
> I had purchased a GPSDO a few weeks before and
> had  been observing its behavior relative to a free-
> running Rb by watching 10 MHz sinewaves drift with
> respect to each other as an aid in setting the Rb's
> frequency.  However, I was seeing enough fairly
> rapid random drift to limit the usefulness of this kind
> of observation.   It dawned on me that I was sometimes
> seeing drifts of several ns over the course of just
> several seconds, thus implying that sometimes the
> relative frequency error between the two sources was
> reaching as high as roughly 1E-9.  I wanted to be able
> to capture and plot a somewhat extended run of data
> so I could try to understand this behavior better.
> Being TIC-less, I decided to see what I could do with
> my o'scope, which is a Chinese-made 2-channel DSO
> with synchronous sampling by the two channels and
> with a respectable trace memory depth (28 MSA per
> channel).
> I began this effort  in earnest a couple of days before I
> saw Dave's question, and have only now brought it to
> a sufficient state of completion to feel justified in reporting
> some results.
> I am presently able to record about 45 minute's worth of
> data as limited by the 'scope's trace memory, but my XP
> computer's RAM space limits me to processing only about
> 35 minutes of that in a seamless run.   Over that time
> span I've seen a peak relative frequency discrepancy of
> about 1.4E-9, with a handful reaching or exceeding 1E-9.
> I've also measured average frequency differences between
> the source's a a few parts in 10E11.
> Most of the effort went into developing a C program to do
> the processing and then correctly scaling and displaying
> the results in a form which I considered useful to me.  This
> processing of course had to deal with an off-frequency and
> drifting 'scope timebase, which is *horrible* compared to the
> quantities under measurement (as expected from the outset).
> Present indications are that at this level of GPSDO mis-
> behavior, the results I'm viewing are about 20 dB higher
> than the basic floor, which I am still characterizing.  I
> believe that the floor is limited primarily by uncorrelated
> sampling jitter between the two 'scope channels.
> If there is an expression of interest in this technique, I'll
> publish a detailed description of the technique and some
> plots showing results, probably in the form of an attachment
> in pdf format.
> Dana
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